Being the leader of the Omega Gang, Callisto dedicated much of her mutant gang's happiness of spreading chaos against humankind due to the latter treating them as outcasts. When news break upon the discovery of a mutant cure, Callisto was very displeased by this as she enjoys her enchanced senses and super-speed and intends to keep it that way.
Deciding that the mutant cure should be destroyed, Callisto met up with Magneto and agreed to aid him and the Brotherhood in tracking down a mutant boy named Leech (whose powers are the source of cure). To that end, Callisto used her powers to track down several more members of the Brotherhood (such as Mystique, Juggernaut, and Pyro) to Magneto's plot, even overpowering Storm in Dark Phoenix's house. They manage to track Leech down to Alcatraz Prison, but the X-Men arrived to the rescue.
During the fight between the X-Men and the Brotherhood, Callisto noticed Storm using her powers once again, so she uses her powers to fight against Storm for a second time. At first, Callisto had the upper hand as her reflexes were enough to take down Storm, but the latter eventually defeats Callisto by throwing her against an electrified fence, electrocuting her to death.